Japanese Society of Grassland Science ISSN1744-6961 SHORTBlackwellMeGRSGr1744-6961©XXXCaseHaR. 2008Ishiwakaasslandrvlbourne, estReports Blackwell mouse PublishingScience andAustralia as Y. Publishing aMasuda control Asia agent Ltd REPORT Possible biological control of the armyworm by the harvest mouse Reiko Ishiwaka and Yasuhisa Masuda Laboratory of Animal Feed Science, Faculty of Agriculture, Kyushu University, Fukuoka, Japan Keywords: Micromys minutus, Mythimna separate, pest control. Received: 22 February 2007; accepted 09 October 2007 Correspondence: Reiko Ishiwaka, Laboratory of Animal Feed Science, Faculty of Agriculture, Kyushu University, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan. Email: [email protected] doi: 10.1111/j.1744-697X.2007.00105.x and Australia, specifically between a latitude of 45°N to 45°S, Introduction and a longitude of 60°E to beyond 170°W (Sharma & Davies The harvest mouse, Micromys minutus Pallas (Rodentia: 1983, cited by Sharma et al. 2002). Larvae of the armyworm Muridae), is highly adept at climbing grass leaves and stems feed on primarily leaves of gramineous plants and periodically with its remarkably small body size (7–9 g in adults), grasping causes serious damage to the following crops: sorghum hands and feet and a prehensile tail; this fact reveals how (Sorghum bicolor [L.] Moench), pearl millet (Pennisetum glaucum effectively this species has adapted to inhabit the stalk zone of [L.] R.Br.), O. sativa, maize (Zea mays L.), wheat (Triticum aestivum Gramineae communities (Ishiwaka & Mõri 1999). The harvest L.) and sugarcane (Saccharum officinarum L.) (Sharma et al. 2002). mouse is widely distributed in the temperate and humid This pest affects the following forage plants as well: perennial climate zone between East Asia and West Europe (Harris & ryegrass (Lolium perenne L.), orchardgrass (Dactylis glomerata Trout 1991). In most of the regions in this zone, Gramineae L.), reed canarygrass (Phalaris arundinacea L.), redtop (Agrostis communities progress to climax vegetation in which trees alba L.), and weeping lovegrass (Eragrostis curvula Nees) are dominant, unless disturbances such as burning, floods, (Watanabe 1961; Koyama & Watanabe 1962; Kanda et al. 1977). grazing or harvesting occur. Despite such a wide distribution, Yield losses are influenced largely by the stage at which the the International Union for Conservation of Nature (IUCN) damage occurs and the gregarious behavior of the larvae. has listed this species in the “near threatened” category since Sometimes, an outbreak results in complete crop loss. Out- 1996 (IUCN 2007). breaks of the armyworm in crop fields have been recorded The harvest mouse appears to be strongly associated with many times in South Asia (Lin et al. 1964; Thakur et al. 1987; agricultural lands including their surrounding grass strips Sharma et al. 2002). In Japan, serious damages to rice and other that are exposed to various disturbances (Harris & Trout 1991; gramineous crops have been reported frequently (Ohmori Bence et al. 2003; Moore et al. 2003; Surmacki et al. 2005). 1960; Koyama 1963, 1966, 1970; Oku & Kobayashi 1974; Oku Furthermore, in Japan, many harvest mice appear to inhabit et al. 1979; Hirai et al. 1985); in contrast, only sporadic outbreaks some gramineous crop fields and their edges (i.e. grass strips); have occurred in agricultural grasslands such as meadows this has occurred following the conversion of many river banks (Oku et al. 1976; Kanda et al. 1977; Kanda 1988). into concrete walls. The unique spherical nests of the harvest The use of insecticides has depressed the populations of mouse are often observed in rice (Oryza sativa L.) communities herbivorous insects to some extent, but their use is severely in paddy fields and in Imperata cylindrica (L.) Beauv. var. restricted, particularly in organic agriculture and forage crop koenigii (Retz.) Durand et Schinz communities in paddy levees. fields. Therefore, the biocontrol of pest insects has been inves- However, rice farmers sometimes remove these nests to prevent tigated in crop fields. Insectivorous birds, parasitic wasps as well damage to their crops. Compared with rice fields, the less inten- as arthropod predators such as spiders, and pathogens have sive management of agricultural grasslands such as meadows been studied in relation to the biocontrol of pest insects (e.g. and pastures allows the harvest mouse to maintain its popu- Fowler et al. 1991; Bock et al. 1992; Donovan 2003; Hooks lation, even though machinery, chemical fertilizer and intro- et al. 2003; Rosa and Simões 2004; Jones et al. 2005). duced crop plants are applied there (Tsukada et al. 2004; Forty-two parasitoids, 15 predators, four bacteria, five R. Ishiwaka, unpubl. data). fungi, and three viral strains have been reported as natural The armyworm, Mythimna separata Walker (Lepidoptera: control agents for the armyworm (Sharma & Davies 1983, Noctuidae), is a pest of several cereal and forage crops in Asia cited by Sharma et al. 2002). The armyworm larvae tend to 52 Grassland Science 54 (2008) 52–56 © 2008 Blackwell Publishing Ltd R. Ishiwaka and Y. Masuda Harvest mouse as a control agent remain hidden under the ground or in the sheath of the host three cages, with one pair in every cage as treated condition, plant during the day, and they feed at night (Kanda 1988). It prior to release of the armyworm. Eighteen days after the is natural that the control agents for the armyworm should placing of the harvest mouse, we examined the guineagrass exclude bird species because most insectivorous ones are leaves in the cages with the harvest mouse to evaluate whether diurnal. Mammal species, on the other hand, usually become it can be a pest of this crop. The armyworm larvae in various more active at night; therefore, they can potentially depress instars were obtained from an infested sudangrass (Sorghum the populations of such nocturnal pest insects. However, to sudanense Piper [Stapf ]) crop field located 4 km east. Thirty-eight the best of our knowledge, studies on any omnivorous or to 40 individuals of the armyworm were placed inside each insectivorous mammal as a natural control agent for pest cage; they were transferred with similar instar composition to insects have not been available to date, except for a study by all the six cages on 1 November (19–21 individuals cage–1), Ellis et al. (2005), probably because mammal species, especially 3 November (4 individuals cage–1) and 19 November (15 rodents, have been generally regarded as little more than pests. individuals cage–1). In the armyworm, third instars can become Grassland scientists have generally discussed wildlife, especially pupae and no longer feed on leaves no earlier than 18 days at wild mammal species, primarily as pests or in relation to 20°C (Okuyama & Tomioka 1963). Therefore, the guineagrass biodiversity. We assumed that some omnivorous and insec- communities would be exposed to infestation by the army- tivorous mammal species including the harvest mouse would worm larvae at all times during the experiment. Water and food have the potential to modify the interaction between grami- were available for the harvest mouse ad libitum; the diet con- neous forage plants and pest insects as well as other control sisted of commercial laboratory mouse chow NMF (Oriental agents in agricultural grasslands. If function of such a mammal Yeast, Tokyo, Japan) along with the following seeds: canary as an extra biocontrol agent for pest insects is suggested, the grass, Chinese, foxtail, common millet, flax, and sunflower. establishment of integrated pest management in grasslands will be promoted. Moreover, with this study, conservation of a threat- Categorization of guineagrass leaves ened species can directly link to agricultural profitability. The aim of this study is to evaluate the effects of the harvest Forty days after placing the first armyworm larvae into the mouse on a gramineous forage plant infested by the army- cages, we sampled approximately 100 guineagrass leaves from worm. Because of the difficulty in establishment of authentic each cage at random. Then, we classified each of the leaves grassland from which only the harvest mouse is completely into one of five categories based on the infestation stage; removed, we adopted cages, in which the harvest mouse, the namely, the extent of consumption by the armyworm in armyworm and a forage grass were brought, on a crop field. terms of leaf area. The categories were as follows: intact leaves (no consumption detected); up to 25% of the leaf area consumed; between 25% and 50% of the leaf area consumed; Materials and methods between 50% and 75% of the leaf area consumed; and more than 75% of the leaf area consumed. The tendency of the Experimental condition armyworm to neglect midribs when feeding on grass leaves We used harvest mice from a colony established with five wild enabled us to estimate the damage it caused to the leaves in individuals captured in Saga, Kyushu, Japan in 1990. In May terms of dry weight. Based on the length of the midrib of a 2001, we built six cages sized 90 cm × 90 cm × 100 cm on the damaged leaf, we estimated the dry weight of the leaf before ground at a crop field in Fukuoka, Kyushu, Japan (5 m a.s.l.). consumption; this was done by means of a relationship curve The cages were made of wire mesh (5 mm × 5 mm) and wood. of the midrib length versus the dry weight of more than 500 Guineagrass (Panicum maximum Jacq.), a kind of tropical intact leaves. The actual dry weight of the damaged leaves forage grass, was seeded inside and outside the cages. The top subtracted from the estimated dry weight before damage is of each cage was covered with a sheet of transparent vinyl equivalent to the yield loss caused by the armyworm. chloride in order to keep the harvest mouse from dying and its artificial food (described later) from decay.